Forgeable corrosion-resisting steel with high neutron-absorption capacity



United States Patent 3,340,047 FORGEABLE CORROSION-RESISTING STEEL WITHHIGH NEUTRON-ABSORPTION CA- PACITY Karl Gerhard Sune Persson, Kedjeasen,Paul Helmer Lindroth, Soderfors, and Lars Ivar Hellner, Karlskoga,Sweden, assignors to Aktiebolaget Bofors, Bofors, Sweden, a corporationof Sweden N0 Drawing. Filed Nov. 26, 1963, Ser. No. 326,216 Claimspriority, application Sweden, Dec. 8, 1962, 13,250/ 62 6 Claims. (Cl.75-126) The present invention relates to a forgeable, corrosionresistingsteel with a high neutron-absorption capacity. Such a steel is wellsuited for the manufacture of control rods for nuclear reactors andradiation-protection screens.

In a nuclear reactor, the development of energy is directly affected bythe flow of neutrons, and it is therefore necessary to be able tocontrol this flow. Such control is usually carried out by insertingcontrol rods into the flow, which have a screening effect, owing totheir containing a substance which has a high absorption crosssectionfor neutrons, and such a substance is boron which, contrary to mostother substances with the corresponding properties, is also cheap. Theactive constituent in boron is the isotope B of which there is a contentof 18.9% in natural boron.

With consideration to the strength required, however, it is not possibleto manufacture control rods of pure boron or any simple chemicalcompound thereof, and it is usually necessary to use boron as analloying element in steel, in order to obtain the strength required.However, the quantity of boron which can be added is limited, owing tothe fact that large boron contents have a very detrimental influence onthe hot-working properties, and when unalloyed steel is used, a boroncontent of not more than approx. 2.5% can be permitted, if satisfactoryforgeability is to be obtained. It is known, however, that by addingaluminum or silicon at the same time, the boron content in a steel alloycan be increased to a maximum of 4.5%, while fairly acceptablehot-working properties are still maintained.

However, steels of the above-mentioned types have comparatively poorcorrosion resistance, and for this reason often cannot be used. Attemptshave therefore been made to use boron as an alloy in a basic materialconsisting of austenitic, stainless steels of the type 18% chrome-8%nickel. In these steels, however, the hotworking properties have beenlost entirely if the boron contents have exceeded 2%.

It has now proved that with a steel according to the present invention,good hot-working properties can be maintained even with boron contentsof up to approx. at the same time as a corrosion resistance is obtainedwhich is considerably better than that of unalloyed steel.

The new forgeable, corrosion-resisting steel with a highneutron-absorption capacity, is characterized according to the presentinvention in that it contains and titanium in such a quantity that, whenaccounted for as weight percent, it amounts to at least 3.1 (weightpercent boron)-5.8

in addition to which the type of steel in question has normal contentsof silicon and manganese, and impurities such as phosphorous andsulphur.

3,340,047 Patented Sept. 5, 1967 In the following table, some examplesare given of steels according to the present invention.

Contents, percent Steel No.

Carbon Chrome Titanium Boron 0. 03 25. l 4. 5 2. 4 0. 04 25. l 6. 0 3. 00. 04 25. l 5. 4 3. 5 l). 03 25. 3 6. 6 3. 7 0. 03 25v 4 6. 8 3. 7 0.(13 25. 4 7. 3 3. 8 0. O3 25. 4 7. 4 4. 0 0. 04 24. 0 8. 0 4. 4

In addition to the constituents mentioned above, the steels also havesomewhat varying contents of silicon and manganese, although not morethan 2% of each, as well as normal impurities such as phosphorous andsulphur.

All of the above-mentioned steels proved to have good hot-workingproperties, and could be forged from ingots into bars without any majordifficulties. The forging was then carried out within the temperaturerange of 950- 1100 C.

All of the steels listed in the table also showed extremely goodcorrosion resistance. In order to obtain the good corrosion resistancedesired it proved necessary to allow the chrome content to amount tomore than 15%.

Keeping the carbon content low, below 0.15%, facilitates thehot-working, and because of the comparatively high chrome content, thesteel will be ferritic within the whole of the temperature intervalwhich will come into question.

The major portion of the boron used should be in the form of titaniumboride, but it is not possible, however, to foresee theoretically theminimum quantity of titanium which is required in order to obtainsatisfactory hot-working properties. The relation between the minimumtitanium content and the boron content has therefore been found as theresult of a great number of experiments.

What is claimed is:

1. Forgeable, corrosion-resisting steel, having a highneutron-absorption capacity, consisting essentially of:

Percent Carbon, maximum 0.15 Chromium 15-30 Boron, maximum 5 Titanium inweight percent of at least 3.l (weight percent boron)-5.8

balance essentially iron.

2. Steel in accordance with claim 1 which contains Percent Silicon,maximum 2 Manganese, maximum 2 References Cited UNITED STATES PATENTS5/1924 Golyer -126 DAVID L. RECK, Primary Examiner.

P. WEINSTEIN, Assistant Examiner.

1. FORGEABLE, CORROSION-RESISTING STEEL, HAVING A HIGHNEUTRON-ABSORPTION CAPACITY, CONSISTING ESSENTIALLY OF: